Extensive development work spanning many decades has been spent in optimizing the design of commercial air conditioners.
One attempt to optimize commercial air conditioners includes the installation of an economizer. Economizers have been commonly used in screw chillers. A typical economizer introduces refrigerant flash gas from an intercooler to the compressor at a pressure that is between that of the evaporator and the condenser. The introduction of moderate pressure refrigerant gas improves the efficiency of the thermodynamic cycle in spite of limited compressor efficiency. In another attempt to optimize commercial air conditioners, a heat exchanger is used to cool the refrigerant liquid leaving the condenser using refrigerant boiling at an intermediate pressure which is returned to the compressor. Both types of systems are commonly used and give a relatively small improvement in capacity (about 10% to about 15%) for typical air-cooled chiller conditions. Another limitation is that these approaches require a special port to the compressor to allow the introduction of intermediate pressure gas. Additional compressor losses associated with this port generally do not allow the full theoretical benefit of the economizer cycle.
Powered subcoolers have seen limited use in low-temperature refrigeration systems, such as in supermarket refrigerators and/or freezers. They use a separate refrigerant circuit for cooling refrigerant liquid in the main refrigeration system in order to obtain lower temperatures at the evaporator required for refrigeration. They have found little or no use in air conditioning systems. These systems generally cool liquid refrigerant to about 32° F. to 50° F. (0 to 10° C.), which would introduce a substantial performance penalty in air conditioning systems. In addition, the prior art teaches the use of separate condensers for the main circuit and subcooler circuit, which increases the space requirements for the system.
With the phase out of HCFC-22 (chlorodifluoromethane), the industry is moving rapidly toward the use of higher-pressure refrigerants. The new refrigerants have pressures higher than that of HCFC-22 with the most promising candidate being designated by ASHRAE as R-410A, a mixture of difluoromethane (R32, CH2F2) and pentafluoroethane (R125, CHF2CF3). This refrigerant has found use in HVAC chiller applications. Other higher-pressure refrigerants include carbon dioxide, R32, and R125. When the outdoor ambient temperature is very high (i.e., greater than about 95° F. (35° C.)), the temperature of refrigerant in the condenser begins to approach its critical temperature. For example, R-410A has a critical temperature of about 160° F. (71° C.). For carbon dioxide with a critical temperature of 90° F. (32.2° C.), the issues with operation near or above the critical temperature are even greater. As the refrigerant reaches or exceeds its critical temperature, the condenser loses the ability to condense the refrigerant, leading to efficiency and capacity losses and/or system failures during times of high outdoor ambient temperature.
What is needed is an HVAC chiller system having improved cooling capacity and efficiency without the drawbacks of the prior art.